It is well known in the earth boring art to produce relatively large diameter holes between a first location and a second location in a mine or other underground works by operations commonly referred to as raise drilling and blind hole drilling. A raise drilling operation begins by drilling a small diameter pilot hole through the earth from a first location to an opening at a second location using a small diameter pilot bit. After the pilot hole is completed, the pilot bit is removed from the drill string and a large diameter raise bit is attached to the drill string. The raise bit is rotated and drawn along the pilot hole thereby enlarging the pilot hole to the desired size. A blind hole drilling operation produces a large diameter hole in a single pass. A pilot hole drill bit is mounted on an extension that precedes the blind hole bit.
Many strict requirements are imposed upon drill bits used in boring large diameter holes. The drill bit must be a balanced, high-performance apparatus that is rugged and will perform for a long period of time. Replaceable rolling cutters are located and spaced so that upon rotation of the drill bit every portion of the hole being drilled will be acted upon by one or more of the cutters in order to disintegrate the formations and form the desired large diameter hole. This insures that almost the entire wear in drilling takes place on the cutters rather than on the main bit body. The cutters are readily replaceable thereby allowing the life of the drill bit to be extended by replacing the individual cutters. Inefficiency and rapid wear is often associated with the cutters in the immediate area of the drive stem. This is due to cutter mounting limitations and the difficulty in positioning the cutting surfaces of the innermost cutters adjacent the drive stem.